The invention relates generally to optics systems (including communications) and optical waveguides and more particularly to the temperature compensation of optical fibers and component/features fiber Bragg gratings.
Fiber Bragg gratings (FBG) have become important components in optical communications systems. FBG are formed by holographic exposure of photosensitive fiber to ultraviolet light, which creates a permanent refractive index grating along the core. The sharp reflection resonances are applied in DWDM (Dense Wavelength Division Multiplex) communication systems. These applications demand that the FBG wavelength does not change with temperature. Unfortunately FBG typically display variations in grating wavelength over temperature of about 0.01 nm/deg C., caused primarily by changes in the refractive index of the fiber, with an additional small contribution from the thermal expansion of the fiber. These coefficient of thermal expansions are clearly undesirable for the narrow channel spacings being used in DWDM systems.
Various methods have been devised for providing temperature independence for the FBG wavelength. These methods range from active systems, which utilize feedback to monitor and dynamically control certain parameters, to passive devices which utilize the thermal characteristics of materials to control the sensitivity of the FBG wavelength to temperature. Passive devices are more desirable since they are much simpler and require no power source. The FBG wavelength is determined by the index of refraction of the fiber and the spacing of the grating, both of which change with temperature. The index of refraction dominates in the sensitivity of wavelength to temperature. However, since the index of refraction is not easily controlled, passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG. This is usually accomplished by clamping the fiber containing the FBG into a mechanical structure made of materials having different, positive coefficient of thermal expansions of expansion. The structure is arranged such that different rates of expansion between the structural members supporting the fiber result in a negative elongation of the fiber with-increasing temperature. Typically the fiber is stretched at low temperature and is allowed to relax as temperature is increased. Examples of this method are described in U.S. Pat. No. 6,044,189 and the White paper xe2x80x9cZero-Wavelength Shift temperature Stabilized Multi-FBG Packagexe2x80x9d by J. J. Pan, S. X. Li, and Y. Shi in E-Tek Dynamics Inc. One disadvantage of the first method is that devices which are somewhat large may be required. Also, such a method may result in bending of the fiber. Bending may introduce undesirable effects such as broadening of the reflection peak, and fiber fatigue. Such frequent bending of the fibers may also cause reliability issues.
Another passive method of temperature compensation involves attaching the fiber containing the FBG to a material having the desired negative coefficient of thermal expansion, such as adopted by Corning and described in a paper presented at the 22nd European Conference (1996) on Optical Communication in Weidman et. al. entitled xe2x80x9cA Novel Negative Expansion ubstrate Material for Athermalizing Fiber Bragg Gratingsxe2x80x9d. A disadvantage of this method (the single-material approach) is that it requires very careful control of the formulation of materials to obtain the desired negative coefficient of thermal expansion of expansion.
It is an object of the invention to obviate or mitigate one or more of the above-identified disadvantages.
Embodiments of the invention provide a temperature-compensating package for an optical feature such as a fiber grating, a fiber arrangement suitable for use in such a temperature compensating arrangement, methods of manufacture for such packages and arrangements, and methods of temperature compensating an optical feature such as a fiber grating.
Advantageously, in the fiber grating temperature compensating package, there is no bending of the fiber grating. There is no stringent requirement on coefficient of thermal expansion of the materials involved. Rather, a precise match can be accomplished by adjusting the length of two sections of material for any fiber. This may be important since typically two batches of fiber will have slightly different characteristics.
According to one broad aspect, the invention provides a fiber holder comprising a first portion made of a first material having a positive coefficient of thermal expansion and a second portion made of a second material having a negative coefficient of thermal expansion, the first and second portions being arranged to form an end-to-end arrangement such that a selected combined coefficient of thermal expansion for the fiber holder results.
The fiber holder may be adapted to cause a change in a length of fiber as a function of temperature which compensates for a change in index of refraction of the length of fiber as a function of temperature. Typically, the fiber holder has a mechanism for imposing a change in a length of fiber as a function of a change in length of the fiber holder. For example, the fiber holder in one embodiment has a longitudinal groove in the end-to-end arrangement of the first and second portions for receiving a fiber.
To hold the fiber in place, the holder may have two opposed surfaces, one surface being in said first portion, the other surface being in said second portion, and adapted to hold a fiber under tension between the two opposed surfaces.
The fiber may have a feature thereon which has a change in behavior as a function of temperature due to changes in the index of refraction of the fiber which requires compensation. The feature in one embodiment is a fiber grating, such as a fiber Bragg grating.
According to another broad aspect, the invention provides a fiber assembly having a first fiber portion with a first cover followed by a second fiber portion without a cover followed by a third fiber portion with a second cover, a first ferrule fixed at one end of the second fiber portion, and a second unfixed ferrule on the second fiber portion. This assembly is suitable for installation on the above-discussed fiber holder.
Another broad aspect provides a method of making a fiber assembly which begins with the provision of two fibers. A cover portion is removed from an end of each of the two fibers to expose two uncovered portion. Two ferrules are placed on the uncovered portions. The uncovered portions are fused together to produce the second fiber portion. Finally, one of the ferrules is fixed in place at one end of the second portion.
The use of ferrules provides a good grip on the fiber, and introduces no birefringence and/or polarization dependent loss.
It is noted that this method is not limited to two materials. With two materials, the first order temperature effect of FBG can be eliminated. With more materials with different thermal coefficients, high order dependence can be compensated.
Another embodiment of the invention provides a stretchable optical transmission medium holder comprising a first portion made of a first material having a positive coefficient of thermal expansion and a second portion made of a second material having a negative coefficient of thermal expansion, the first and second portions being arranged to form an end-to-end arrangement such that a selected combined coefficient of thermal expansion for the stretchable optical transmission medium holder results.